The 6-foot-10 (2.08-meter) American made sure of that Saturday, rallying to beat seventh-seeded Juan Martin del Potro 6-7 (4), 7-6 (9), 6-3 in a grueling Western &amp; Southern Open semifinal.

Written by Associated Press

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Unseeded American John Isner rallied from losing a first-set tiebreaker to upset seventh-seeded Juan Martin del Potro 6-7 (4), 7-6 (9), 6-3 in a grueling Western & Southern Open semifinal of Saturday.

Isner, who fell from No. 20 to 22 this week, will return to the top 20 after a stimulating Cincinnati run that includes beating three straight top-10 players for the first time in his career while earning his second Masters 1000 series finals appearance and first in Cincinnati. His previous Masters 1000 final appearance was at Indian Wells in 2012.

"It's awesome to be at this stage again," said Isner, who beat top-ranked Novak Djokovic in the quarterfinals and No. 8 Richard Gasquet in the second round. "I've been playing pretty well this season, but this tournament is where I've really started to put it all together. I'm playing pretty well in all facets of the game."

Isner, in his first appearance in the tournament's semifinals, had been 0-4 against the Argentine. He'll play third-seeded Rafael Nadal, who beat sixth-seeded Tomas Berdych 7-5, 7-6 (4) in the other semifinal to reach the tournament final for the first time in nine appearances.

"This is one tournament that I've never been able to play well, so it's something special to play well in a place I never did in the past," said Nadal, 3-0 in his career against Isner. "I'm very happy for that. I am having the right feelings on court. I'm playing aggressive, and the crowd here is great."

Nadal extended his personal winning streak against the Czech to 14 matches while improving his overall season record 52-3, including 14-0 on hardcourts. After winning at Montreal last week, the Spaniard will be making back-to-back appearances in hardcourt finals for the first time in his career.

He's 15-1 against top-10 players this season.

Top-ranked Serena Williams also will be making her first career appearance in the event's championship match after ousting defending champion Li Na, 7-5, 7-5, in the first women's semifinal. Williams, seeking her ninth title of the year, advanced when Li double-faulted on match point. Williams improved to 60-3 this season.

"I really would not have predicted myself being in the finals here at all, so it's definitely pretty exciting for me," said Williams, who clinched the 2013 Emirates Airline U.S. Open Series women's title with the win and can earn a $1 million bonus if she wins the U.S. Open. "Really unexpected for me, too. I came in here not playing my best, and now I'm in the final. It's just almost weird."

Williams will play second-seeded Victoria Azarenka in Sunday's final. Azarenka beat 14th-seeded Jelena Jankovic 4-6, 6-2, 6-3 in a semifinal that featured 23 service breaks in 27 games.

Isner needed 2 hours, 47 minutes to improve to 16-3 since retiring from Wimbledon in the second round because of a left knee injury. He finished off the win when del Potro sent a backhand from the baseline into the net.

Despite tossing into a persistent swirling breeze and bright sunshine, both players served so effectively early that the first set included just one break point, fought off by the 22nd-ranked Isner. The 6-foot-6 Del Potro came up with a crucial mini-break on the 11th point of the tiebreaker when Isner sailed his backhand volley wide to lose the set.

Del Potro broke Isner in the eighth game of the second set to put himself into position to serve for the match, but he double-faulted on match point and Isner came up with his own break to stay alive and eventually force a tiebreak that he won when del Potro sent a forehand wide to the deuce court.

"I think I lost the match in the second set," del Potro said. "That was my chance to beat him. He was fighting all the time and deserved to win the third set, but I missed my chance."

"I know, at that moment, that the sun on that side was pretty bad," said Isner, who can become the first American to win the tournament since Andy Roddick in 2006. "We both struggled with that for about 30 minutes."

Despite the loss to Nadal, Berdych still will move up a notch in the rankings to a career-high fifth, passing Roger Federer.